To Catch a Pulsar by the Tail

Pulsars—rapidly spinning stellar remnants that cram the mass of the sun into a ball 10 miles across—are natural physics labs. Their extreme properties test the limits of our theories of gravity and magnetism. Giovanni Bignami, an astrophysicist at the Centre d’Etude Spatiale des Rayonnements in Toulouse, France, has now found a pulsar that tests physics another way, by illuminating the invisible stretches of interstellar material with a brilliant blast of energetic electrons.

Bignami was studying images of the Geminga pulsar taken by the XMM-Newton X-ray Observatory when

Courtesy of Giovanni Bignami

he discovered a strange double tail streaking 1.9 trillion miles behind the star. No one had ever seen such a thing before. He realized that the tails could act as a tracer, making it possible to deduce exactly how the pulsar is moving through space: 75 miles per second to the northwest, almost directly across our line of sight. The tails also act as probes of the thin, electrically charged gas that fills space. “The pulsar moves like a cannonball through the interstellar medium at 20 times the speed of sound, creating a bow shock like a sonic boom. The bow shock acts as a kind of magnetic funnel trapping electrons, which then shine in X rays,” Bignami says. By modeling that shock, he could measure the weak magnetic field that spans stars. Geminga is the only object known to emit the high-energy electrons needed to create such tails. Bignami is now hunting for more.